Radiation is commonly used in the non-invasive inspection of objects such as luggage, bags, briefcases, and the like, to identify hidden contraband at airports and public buildings. The contraband may include hidden guns, knives, explosive devices and illegal drugs, for example. As criminals and terrorists have become more creative in the way they conceal contraband, the need for more effective non-invasive inspection techniques has grown. While the smuggling of contraband onto planes in carry-on bags and in luggage has been a well-known, on-going concern, a less publicized but also serious threat is the smuggling of contraband across borders and by boat in large cargo containers. Only 2%-10% of the 17 million cargo containers brought to the United States by boat are inspected. “Checkpoint Terror”, U.S. News and World Report, Feb. 11, 2002, p. 52.
FIG. 1 is a schematic diagram of a radiation scanning system 10 including a radiation source 12 scanning an object 14 with a vertically diverging fan beam 16 of radiation. A detector 18 is behind the object 14, to detect radiation transmitted through the object. The object is moved horizontally (out of the page) through the vertically extending fan beam 16 by a conveying system (not shown). Radiation transmitted through the object 14 is attenuated to varying degrees by the object and its contents. The attenuation of the radiation is a function of the density and atomic composition of the materials through which the radiation passes. The attenuated radiation is detected and radiographic images of the contents of the object 14 are generated for inspection. The images show the shape, size and varying densities of the contents.
To examine larger objects (greater than about 5 feet, 1.5 meters thick, for example), the radiation source 10 may be a linear accelerator including a source of electrons 20 and a target 22 of material having a high atomic number, such as tungsten. An electron beam 24 is shown being emitted along an axis R through the electron source 20 and the target 22, referred to as a central ray. The electron beam 24 impacts the target 22, causing generation of a beam of X-ray radiation. Linear accelerators are described in more detail in U.S. Pat. No. 6,366,021 B1, U.S. Pat. No. 4,400,650 and U.S. Pat. No. 4,382,208, which are assigned to the assignee of the present invention and are incorporated by reference, herein.
The radiation beam is collimated into the fan beam 16 by a collimator (not shown) at a distal end of the source 12. The fan beam 16 is emitted over an arc of about 30°. The fan beam illuminates a front face 14a of the object 14. The system 10 may be referred to a line scanner.
The intensity of the X-ray beam at point A on the face of the object 14, aligned with the central ray, is at a maximum M. The intensity of the X-ray beam 18 decreases as the angle from the central ray R increases. At best, the intensity is substantially uniform over only a few degrees around the central ray. For example, for a 9 MeV (peak intensity) X-ray beam 18, the intensity of the beam at an angle of about +/−12°, indicated as points B and C on the face 11a of the cargo conveyance 18, is about 50% of the intensity at point A, along the central ray. Better radiation scanning systems for scanning objects can compensate for intensity drops of up to about 50%. As intensity drops beyond about 50% at higher angles from the central ray R, however, the object penetration and contrast sensitivity may become significantly reduced. The intensity of the radiation beam also decreases as the distance between the source 10 and the object 14 increases, as a function of the square of the distance.
Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters) and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Sea cargo containers are typically about 40 feet long, 8 feet wide and 8 feet high. Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side wall, may be of comparable sizes as cargo containers. The term “cargo conveyance” is used herein to encompass cargo containers, sea containers and pallets.
To illuminate large cargo conveyances with a more uniform portion of an X-ray beam (within about 50% of maximum), the source must be very far from the cargo conveyance. For example, to illuminate a cargo container with a height of about 8 feet (2.4 meters) with a vertical radiation beam emitted over an angle of about 24 degrees (+/−12 degrees from the central ray), the source needs to be about 19 feet (about 5.8 meters) from the face of the cargo container. If the beam could be emitted over an angle of about 120 degrees (+/−60) degrees from the central ray), in contrast, the source may be about 2.5 feet (about 0.8 meters) from the face of the cargo container. More compact radiation scanning systems, where the radiation source is closer to the object than in current systems, would be advantageous. They would occupy less space, as well as suffer from less drop in radiation intensity due to the distance between the source and the object.